The field of art to which the invention pertains includes the art of acoustics and particularly to mufflers for noise level reduction from the exhaust of rotary-type pneumatic hand tools.
By and large, the majority of commercially available pneumatic hand tools include a muffler of sorts for maintaining the noise level of the air exhaust within tolerable limits. Having to tolerate too high a noise level for any period of time can have a deleterious effect on the workman operating the tool. With the advent of OSHA (Occupational Safety and Health Administration) Federal regulations now impose even stricter standards for such tools than have existed previously rendering the objective of increased muffling not only a basic comfort but a legal requirement. As set forth in the Federal Register, Vol. 39, No. 125 of June 27, 1974, Table G-16 establishes the daily work duration at 8 hours with a noise level of 90 dBA and only 4 hours with a noise level of 95 dBA further decreasing to only 1 hour at a noise level of 105 dBA.
In any pneumatic tool, expansion of the inlet air from a high pressure at the intake to low pressure at the exhaust causes a drop in temperature to occur such that moisture present in the air stream tends to freeze and create an iced over condition on the surfaces over which it passes. For ungoverned pneumatic tools, the icing tendency is minimal and it is customary to employ sound absorbing compositions for the muffler such as cintered metal, metal wool, felt, screen wire, etc. located in the exhaust passage for absorbing noise from the exhausting air before being discharged to atmosphere. With governed pneumatic tools utilizing a governor to maintain control over output speed, the air quantity varies with load and for which such compositions are generally unsuitable because of the greater tendency toward icing of moisture content. Where icing occurs in these tools, it tends to clog or partially block the interstices of the muffler composition reducing flow capacity and inhibiting tool performance. For that reason, governed pneumatic tools typically employ a muffler formed of large cavities which function as a resonator. While effecting and meeting the objective of noise reduction, the resonator type muffler unfortunately contributes significant added size and weight which essentially prohibits their use on a portable-type hand tool. Despite recognition of the problem, a ready solution has not heretofore been known.